Semiconductor Nanoparticle-Based Materials

ABSTRACT

The present invention relates to a primary particle comprised of a primary matrix material containing a population of semiconductor nanoparticles, wherein each primary particle further comprises an additive to enhance the physical, chemical and/or photo-stability of the semiconductor nanoparticles. A method of preparing such particles is described. Composite materials and light emitting devices incorporating such primary particles are also described.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/496,020, filed Sep. 25, 2014, which is a continuation of U.S. patentapplication Ser. No. 12/888,881, filed Sep. 23, 2010, now U.S. Pat. No.8,847,197, which claims the benefit of and priority to GB ApplicationNo. 0916699.2, filed Sep. 23, 2009 and U.S. Provisional PatentApplication Ser. No. 61/246,287, filed Sep. 28, 2009, the disclosures ofwhich are incorporated herein by reference in their entireties.

FIELD OF INVENTION

The present invention relates to semiconductor nanoparticle-basedmaterials, particularly, but not exclusively, quantum dot-containingbeads for use in the fabrication of quantum dot-based light emittingdevices.

There has been substantial interest in exploiting the properties ofcompound semiconductors consisting of particles with dimensions in theorder of 2-50 nm, often referred to as quantum dots (QDs) ornanocrystals. These materials are of commercial interest due to theirsize-tuneable electronic properties which can be exploited in manycommercial applications such as optical and electronic devices and otherapplications that ranging from biological labeling, photovoltaics,catalysis, biological imaging, LEDs, general space lighting andelectroluminescent displays amongst many new and emerging applications.

The most studied of semiconductor materials have been the chalcogenidesII-VI materials namely ZnS, ZnSe, CdS, CdSe, CdTe; most noticeably CdSedue to its tuneability over the visible region of the spectrum.Reproducible methods for the large scale production of these materialshave been developed from “bottom up” techniques, whereby particles areprepared atom-by-atom, i.e. from molecules to clusters to particles,using “wet” chemical procedures.

Two fundamental factors, both related to the size of the individualsemiconductor nanoparticle, are responsible for their unique properties.The first is the large surface to volume ratio; as a particle becomessmaller, the ratio of the number of surface atoms to those in theinterior increases. This leads to the surface properties playing animportant role in the overall properties of the material. The secondfactor being, with many materials including semiconductor nanoparticles,that there is a change in the electronic properties of the material withsize, moreover, because of quantum confinement effects the band gapgradually becomes larger as the size of the particle decreases. Thiseffect is a consequence of the confinement of an ‘electron in a box’giving rise to discrete energy levels similar to those observed in atomsand molecules, rather than a continuous band as observed in thecorresponding bulk semiconductor material. Thus, for a semiconductornanoparticle, because of the physical parameters, the “electron andhole”, produced by the absorption of electromagnetic radiation, aphoton, with energy greater than the first excitonic transition, arecloser together than they would be in the corresponding macrocrystallinematerial, moreover the Coulombic interaction cannot be neglected. Thisleads to a narrow bandwidth emission, which is dependent upon theparticle size and composition of the nanoparticle material. Thus,quantum dots have higher kinetic energy than the correspondingmacrocrystalline material and consequently the first excitonictransition (band gap) increases in energy with decreasing particlediameter.

Core semiconductor nanoparticles, which consist of a singlesemiconductor material along with an outer organic passivating layer,tend to have relatively low quantum efficiencies due to electron-holerecombination occurring at defects and dangling bonds situated on thenanoparticle surface which can lead to non-radiative electron-holerecombination. One method to eliminate defects and dangling bonds on theinorganic surface of the quantum dot is to grow a second inorganicmaterial, having a wider band-gap and small lattice mismatch to that ofthe core material epitaxially on the surface of the core particle, toproduce a “core-shell” particle. Core-shell particles separate anycarriers confined in the core from surface states that would otherwiseact as non-radiative recombination centers. One example is a ZnS shellgrown on the surface of a CdSe core. Another approach is to prepare acore-multi shell structure where the “electron-hole” pair is completelyconfined to a single shell layer consisting of a few monolayers of aspecific material such as a quantum dot-quantum well structure. Here,the core is of a wide band gap material, followed by a thin shell ofnarrower band gap material, and capped with a further wide band gaplayer, such as CdS/HgS/CdS grown using substitution of Hg for Cd on thesurface of the core nanocrystal to deposit just a few monolayers of HgSwhich is then over grown by a monolayer of CdS. The resulting structuresexhibit clear confinement of photo-excited carriers in the HgS layer. Toadd further stability to quantum dots and help to confine theelectron-hole pair one of the most common approaches is by epitaxiallygrowing a compositionally graded alloy layer on the core this can helpto alleviate strain that could otherwise led to defects. Moreover for aCdSe core in order to improve structural stability and quantum yield,rather growing a shell of ZnS directly on the core a graded alloy layerof Cd_(1-x)Zn_(x)Se_(1-y)S_(y) can be used. This has been found togreatly enhance the photoluminescence emission of the quantum dots.

Doping quantum dots with atomic impurities is an efficient way also ofmanipulating the emission and absorption properties of the nanoparticle.Procedures for doping of wide band gap materials, such as zinc selenideand zinc sulfide, with manganese and copper (ZnSe:Mn or ZnS:Cu), havebeen developed. Doping with different luminescence activators in asemiconducting nanocrystal can tune the photoluminescence andelectroluminescence at energies even lower than the band gap of the bulkmaterial whereas the quantum size effect can tune the excitation energywith the size of the quantum dots without having a significant change inthe energy of the activator related emission.

The widespread exploitation of quantum dot nanoparticles has beenrestricted by their physical/chemical instability and incompatibilitywith many of the materials and/or processes required to exploit thequantum dots to their full potential, such as incorporation intosolvents, inks, polymers, glasses, metals, electronic materials,electronic devices, bio-molecules and cells. Consequently, a series ofquantum dot surface modification procedures has been employed to renderthe quantum dots more stable and compatible with the materials and/orprocessing requirements of a desired application.

A particularly attractive potential field of application for quantumdots is in the development of next generation light-emitting diodes(LEDs). LEDs are becoming increasingly important in modern day life andit is envisaged that they have the potential to become one of the majorapplications for quantum dots, in for example, automobile lighting,traffic signals, general lighting, liquid crystal display (LCD)backlighting and display screens.

Currently, LED devices are made from inorganic solid-state compoundsemiconductors, such as AlGaAs (red), AlGaInP (orange-yellow-green), andAlGaInN (green-blue), however, using a mixture of the availablesolid-state compound semiconductors, solid-state LEDs which emit whitelight cannot be produced. Moreover, it is difficult to produce “pure”colors by mixing solid-state LEDs of different frequencies. Therefore,currently the main method of color mixing to produce a required color,including white, is to use a combination of phosphorescent materialswhich are placed on top of the solid-state LED whereby the light fromthe LED (the “primary light”) is absorbed by the phosphorescent materialand then re-emitted at a different frequency (the “secondary light”),i.e. the phosphorescent materials down convert the primary light to thesecondary light. Moreover, the use of white LEDs produced by phosphordown-conversion leads to lower cost and simpler device fabrication thana combination of solid-state red-green-blue LEDs.

Current phosphorescent materials used in down converting applicationsabsorb UV or mainly blue light and converts it to longer wavelengths,with most phosphors currently using trivalent rare-earth doped oxides orhalophosphates. White emission can be obtained by blending phosphorswhich emit in the blue, green and red regions with that of a blue or UVemitting solid-state device. i.e. a blue light emitting LED plus a greenphosphor such as, SrGa₂S₄:Eu₂ ⁺, and a red phosphor such as, SrSiEu₂ ⁺or a UV light emitting LED plus a yellow phosphor such as, Sr₂P₂O₇:Eu₂⁺;Mu₂ ⁺, and a blue-green phosphor. White LEDs can also be made bycombining a blue LED with a yellow phosphor, however, color control andcolour rendering is poor when using this methodology due to lack oftuneability of the LEDs and the phosphor. Moreover, conventional LEDphosphor technology uses down converting materials that have poor colourrendering (i.e. color rendering index (CRI)<75).

Rudimentary quantum dot-based light emitting devices have been made byembedding colloidally produced quantum dots in an optically clear (orsufficiently transparent) LED encapsulation medium, typically a siliconeor an acrylate, which is then placed on top of a solid-state LED. Theuse of quantum dots potentially has some significant advantages over theuse of the more conventional phosphors, such as the ability to tune theemission wavelength, strong absorption properties and low scattering ifthe quantum dots are mono-dispersed.

For the commercial application of quantum dots in next-generation lightemitting devices, the quantum dots should be incorporated into the LEDencapsulating material while remaining as fully mono-dispersed aspossible and without significant loss of quantum efficiency. The methodsdeveloped to date are problematic, not least because of the nature ofcurrent LED encapsulants. Quantum dots can agglomerate when formulatedinto current LED encapsulants thereby reducing the optical performanceof the quantum dots. Moreover, even after the quantum dots have beenincorporated into the LED encapsulant, oxygen can still migrate throughthe encapsulant to the surfaces of the quantum dots, which can lead tophoto-oxidation and, as a result, a drop in quantum yield (QY).

In view of the significant potential for the application of quantum dotsacross such a wide range of applications, including but not limited to,quantum dot-based light emitting devices, work has already beenundertaken by various groups to try to develop methods to increase thestability of quantum dots so as to make them brighter, more long-livedand/or less sensitive to various types of processing conditions. Forexample, reasonably efficient quantum dot-based light emitting devicescan be fabricated under laboratory conditions building on currentpublished methods, however, there remain significant challenges to thedevelopment of quantum dot-based materials and methods for fabricatingquantum dot-based devices, such as light emitting devices, on aneconomically viable scale and which would provide sufficiently highlevels of performance to satisfy consumer demand.

SUMMARY

An object of the present invention is to obviate or mitigate one or moreof the above problems with semiconductor nanoparticle-based materialsand/or current methods for fabricating such materials.

A first aspect of the present invention provides a primary particlecomprised of a primary matrix material containing a population ofsemiconductor nanoparticles, wherein each primary particle furthercomprises an additive to enhance the physical, chemical and/orphoto-stability of the semiconductor nanoparticles.

The current invention thus provides a means by which the robustness, andconsequently, the performance of semiconductor nanoparticles can beimproved for use in a wide range of applications, particularly, but notexclusively the fabrication of semiconductor nanoparticle-based lightemitting devices, preferably where the device incorporates an LED as aprimary light source and the semiconductor nanoparticles as a secondarylight source. By providing each primary particle with one or morestability enhancing additives, the semiconductor nanoparticles are lesssensitive to their surrounding environment and subsequent processingsteps.

In a preferred embodiment a plurality of quantum dots are incorporatedinto one or more silica beads which also include a free-radicalscavenger, such as benzophenone or a derivative thereof, which quantumdot-containing beads are then embedded or entrapped within a host LEDencapsulation material such as a silicone, an epoxy resin, a(meth)acrylate or a polymeric material. Such an arrangement is depictedschematically in FIG. 1, wherein an LED 1, which is arranged to emitblue primary light 2 upon the application of current, is submerged in acommercially available LED encapsulant 3 in which is embedded aplurality of quantum dot-containing silica beads 4, 5 also incorporatinga free-radical scavenger to enhance the stability of the beads; some ofthe beads 4 contain quantum dots that emit red secondary light 6 uponexcitation by the blue primary light from the LED 1, and the remainingbeads 4 contain quantum dots which emit green secondary light 7 uponexcitation by the blue primary light from the LED 1.

The term “bead” is used herein for convenience and is not intended toimpose any particular size or shape limitation to the material describedas a “bead”. Thus, for example, the beads may be spherical but otherconfigurations are possible. Where reference is made herein to“microbeads” this is intended to refer to “beads” as defined abovehaving a dimension on the micron scale.

The, or each, primary particle may be provided with a separate layer ofa surface coating material. The term “coating” is used herein to referto one or more layers of material provided on another material, whichmay partially or fully cover the exterior surface or solvent accessiblesurface of that other material. The material of the “coating” maypenetrate at least partially into the internal structure of the materialto which it has been applied, provided the coating still affords a levelof protection or functions in some way as a barrier to the passage ofpotentially harmful species, e.g. oxygen, through the coated material.It will be appreciated from the wording used to define the variousaspects of the present invention herein that the “coating” applied toeach primary particle results in the production of a plurality ofseparate, distinct coated particles rather than a plurality of particlescontained or encapsulated within the same, unitary matrix-type material,such as a plurality of resin beads dispersed throughout an LEDencapsulant.

The nanoparticle-containing primary particles or beads are preferablyprovided in the form of microbeads. By pre-loading small microbeads,which can range in size from 50 nm to 500 μm or more preferably 25 nm to0.1 mm or more preferably still 20 nm to 0.5 mm in diameter, withquantum dots, adding an additive, and then optionally providing asurface coating of, for example, a polymer or oxide material, theresulting beads are more stable towards their surrounding environmentand/or subsequent processing conditions, such as the incorporation ofthe quantum dot-containing beads into an LED encapsulation material on aUV or blue LED. As a result, not only does handling of the quantum dotsbecome easier, but their optical performance can be improved and it canbecome simpler to tune the color of the light they emit, for examplewhen used in an LED-based device. Moreover, this approach is simplerthan attempting to incorporate quantum dots directly into an LEDencapsulate (for example, a silicone, an epoxy, a (meth)acrylate, apolymeric material or the like) in terms of ease of color rendering,processing, and reproducibility and offers greater quantum dot stabilityto photo-oxidation.

The quantum dot-containing beads can be made to any desirable size, suchas the same size as currently employed YAG phosphor materials whichrange from 10 to 100 μm and can thus be supplied to existing LEDmanufacturers in a similar form to that of the current commercially usedphosphor materials. Moreover, the quantum dot-containing beadsincorporating the additive(s) is (are) in a form that is compatible withthe existing LED fabrication infrastructure.

With the advantage of very little or no loss of quantum dot quantumyield (QY) in processing; this new approach of optionally coated quantumdot-containing beads incorporating stability-enhancing additives leadsto less loss of quantum efficiency than when formulating the quantumdots directly into a LED encapsulation medium or when using quantum dotbeads not incorporating such additives or a protective surface coating.Because there is very little or no loss of quantum yield it is easier tocolor render and less binning is required. It has been shown that whenformulating quantum dots directly into an encapsulation medium usingprior art methods, color control is very difficult due to quantum dotre-absorption or loss of quantum yield and shifting of the PL maxposition. Moreover batch to batch, i.e. device-to-device,reproducibility is very difficult or impossible to achieve. Bypre-loading the quantum dots into one or more beads also incorporatingthe stability-enhancing additive(s), and then optionally coating thebeads, the color of the light emitted by the device is of higherquality, easier to control and is more reproducible.

By incorporating known amounts of quantum dots into beads alsoincorporating stability-enhancing additives, and optionally providingthe beads with a protective surface coating, migration of deleteriousspecies, such as moisture, oxygen and/or free radicals, is eliminated orat least reduced, thereby eliminating or at least minimizing thesecommon hurdles to the industrial production of quantum dot basedmaterials and devices.

A second aspect of the present invention provides a method for preparinga primary particle comprised of a primary matrix material, a populationof semiconductor nanoparticles and an additive to enhance the physical,chemical and/or photo-stability of the semiconductor nanoparticles, themethod comprising combining said semiconductor nanoparticles, primarymatrix material and additive under conditions suitable to produce saidprimary particle.

An additive may be combined with “naked” semiconductor nanoparticles andprecursors to the primary matrix material during initial production ofthe primary particles. Alternatively, or additionally, an additive maybe added after the semiconductor nanoparticles have been entrappedwithin the primary matrix material.

The quantum-dot containing primary particles incorporating an additivecan be dispersed in a secondary matrix material, which may be the sameor different to the primary matrix material.

A further aspect of the present invention provides a composite materialincorporating a plurality of primary particles according to the firstaspect of the present invention dispersed within a secondary matrixmaterial.

A still further aspect provides a light emitting device including aprimary light source in optical communication with a formulationcomprising a composite material according to the above further aspectembedded in a host light emitting diode encapsulation medium.

The secondary matrix material may be selected from the group of primarymatrix materials set out above. By way of example, the secondary matrixmaterial may comprise a material selected from the group consisting of apolymer, a resin, a monolith, a glass, a sol gel, an epoxy, a siliconeand a (meth)acrylate.

Additionally, the secondary matrix material may be formed into one ormore secondary particles containing one or more primary particles. Thesecondary particles may be provided with a further additive in a similarmanner to that described herein in respect of additives added to theprimary particles. Accordingly, the secondary matrix material may be inthe form of one or more secondary particles and the, or each, secondaryparticle may be provided with a further stability-enhancing additive,which may be the same or different to the one or more additives presentin the primary particles.

Alternatively, the quantum dots may first be captured within one or moretypes of matrix material, such as one or more types of polymeric bead,and then each of those beads, or beads within beads, may be containedwithin a primary matrix material to form the primary particles of thefirst and second aspects of the present invention, which incorporate astability-enhancing additive. Thus, the semiconductor nanoparticlescontained within the primary matrix material may be “naked”nanoparticles, or may already be contained within one or more layers ofmatrix material before being captured within the primary matrix materialand coated.

FIGS. 7 to 10 depict processes according to four preferred embodimentsof the present invention in which additives are added at differentstages during the formation of quantum dot-containing beads, or quantumdot-containing beads within one or more types of larger bead.

FIG. 7 illustrates a process according to a first embodiment of thepresent invention wherein an additive is combined with a population of“naked” quantum dots during formation of a primary particle containingthe quantum dots and, consequently, the additive. FIG. 8 illustrates aprocess according to a second embodiment wherein “naked” quantum dotsare first encapsulated within a bead formed of a first type of polymer(polymer 1) and then an additive is combined with the quantumdot-containing bead during formation of a primary particle made from asecond type of polymer (polymer 2) containing the quantum dot-containingbead and, consequently, the additive. FIG. 9 depicts a process accordingto a third embodiment wherein quantum dots are first encapsulated withina population of beads formed of a first type of polymer (polymer 1) andthen an additive is combined with the quantum dot-containing beadsduring formation of a primary particle made from a second type ofpolymer (polymer 2) containing the quantum dot-containing beads and,consequently, the additive. FIG. 10 illustrates a process according to afourth embodiment wherein quantum dots are first encapsulated within apopulation of beads formed of a first type of polymer (polymer 1), whichare then encapsulated within a bead formed of a second type of polymer(polymer 2) to form a “bead-in-bead” composite material, and then anadditive is combined with the quantum dot-containing bead-in-beadcomposite material during formation of a primary particle made from athird type of polymer (polymer 3) containing the quantum dot-containing“bead-in-bead” composite material and, consequently, the additive. Itwill be appreciated that any of the above embodiments may be combinedsuch that additives could be added at more than one stage during primaryparticle formation, resulting in primary particles containingbead-in-bead composites with the same or different additives in two ormore layers or shells of the primary particles.

A further aspect of the present invention provides a light emittingdevice including a primary light source in optical communication with aformulation comprising a plurality of primary particles according to thefirst aspect of the present invention embedded in a host light emittingdiode encapsulation medium.

Primary Matrix Material

The primary matrix material is preferably an optically transparentmedium, i.e. a medium through which light can pass, and which may be,but does not have to be substantially optically clear. The primarymatrix material, preferably in the form of a bead or microbead, may be aresin, polymer, monolith, glass, sol gel, epoxy, silicone,(meth)acrylate or the like.

Examples of preferred primary matrix materials include acrylate polymers(e.g. polymethyl(meth)acrylate, polybutylmethacrylate,polyoctylmethacrylate, alkylcyanoacryaltes, polyethyleneglycoldimethacrylate, polyvinylacetate etc), epoxides (e.g., EPOTEK 301 A+BThermal curing epoxy, EPOTEK OG112-4 single pot UV curing epoxy, orEX0135A and B Thermal curing epoxy), polyamides, polyimides, polyesters,polycarbonates, polythioethers, polyacrylonitryls, polydienes,polystyrene polybutadiene copolymers (Kratons), pyrelenes,poly-para-xylylene (parylenes), silica, silica-acrylate hybrids,polyetheretherketone (PEEK), polyvinylidene fluoride (PVDF), polydivinylbenzene, polyethylene, polypropylene, polyethylene terephthalate (PET),polyisobutylene (butyl rubber), polyisoprene, and cellulose derivatives(methyl cellulose, ethyl cellulose, hydroxypropylmethyl cellulose,hydroxypropylmethylcellulose phthalate, nitrocellulose), andcombinations thereof.

Stability-Enhancing Additives

The additives which may be added singly or in any desirable combinationto the primary particles containing the semiconductor nanoparticles canbe grouped according to their intended function as follows:

-   -   a. Mechanical sealing: Fumed silica (e.g., Cab-O-Sil™), ZnO,        TiO₂, ZrO, Mg stearate, Zn Stearate, all used as a filler to        provide mechanical sealing and/or reduce porosity;    -   b. Capping agents: Tetradecyl phosphonic acid (TDPA), oleic        acid, stearic acid, polyunsaturated fatty acids, sorbic acid. Zn        methacrylate, Mg stearate, Zn Stearate, isopropyl myristate.        Some of these have multiple functionality and can act as capping        agents, free radical scavengers and/or reducing agents;    -   c. Reducing agents: Ascorbic acid palmitate, alpha tocopherol        (vitamin E), octane thiol, butylated hydroxyanisole (BHA),        butylated hydroxytoluene (BHT), gallate esters (propyl, lauryl,        octyl and the like), and a metabisulfite (e.g. the sodium or        potassium salt);    -   d. Free radical scavengers: benzophenones; and    -   e. Hydride reactive agents: 1,4-butanediol, 2-hydroxyethyl        methacrylate, allyl methacrylate, 1,6 heptadiene-4-ol, 1,7        octadiene, and 1,4 butadiene.

The selection of the additive or additives for a particular applicationwill depend upon the nature of the semiconductor nanoparticle material(e.g. how sensitive the nanoparticle material is to physical, chemicaland/or photo-induced degradation), the nature of the primary matrixmaterial (e.g. how porous it is to potentially deleterious species, suchas free-radicals, oxygen, moisture etc.), the intended function of thefinal material or device which will contain the primary particles (e.g.the operating conditions of the material or device), and the processconditions required to fabricate said final material or device. Thus,with prior knowledge of the above risk-factors, one or more appropriateadditives can be selected from the above five lists to suit anydesirable semiconductor nanoparticle application.

Primary Particle Surface Coating Materials

One of the intended functions of the coating which may be provided onthe primary particles is to provide each primary particle with aprotective barrier to prevent the passage or diffusion of potentiallydeleterious species, e.g. oxygen, moisture or free radicals, from theexternal environment through the primary matrix material to thesemiconductor nanoparticles. As a result, the semiconductornanoparticles are less sensitive to their surrounding environment andthe various processing conditions typically required to utilize thenanoparticles in applications such as the fabrication of LED-based lightemitting devices.

The coating is preferably a barrier to the passage of oxygen or any typeof oxidizing agent through the primary matrix material. The coating maybe a barrier to the passage of free radical species through the primarymatrix material, and/or is preferably a moisture barrier so thatmoisture in the environment surrounding the primary particles cannotcontact the semiconductor nanoparticles within the primary particles.The coating may provide a layer of coating material on a surface of theprimary particle of any desirable thickness provided it affords therequired level of protection. The surface layer coating may be around 1to 10 nm thick, up to around 400 to 500 nm thick, or more. Preferredlayer thicknesses are in the range 1 nm to 200 nm, more preferablyaround 5 to 100 nm.

In a first preferred embodiment, the coating comprises an inorganicmaterial, such as a dielectric (insulator), a metal oxide, a metalnitride or a silica-based material (e.g. a glass).

The metal oxide may be a single metal oxide (i.e. oxide ions combinedwith a single type of metal ion, e.g. Al₂O₃), or may be a mixed metaloxide (i.e. oxide ions combined with two or more types of metal ion,e.g. SrTiO₃). The metal ion(s) of the (mixed) metal oxide may beselected from any suitable group of the periodic table, such as group 2,13, 14 or 15, or may be a transition metal, d-block metal, or lanthanidemetal.

Preferred metal oxides are selected from the group consisting of Al₂O₃,B₂O₃, Co₂O₃, Cr₂O₃, CuO, Fe₂O₃, Ga₂O₃, HfO₂, In₂O₃, MgO, Nb₂O₅, NiO,SiO₂, SnO₂, Ta₂O₅, TiO₂, ZrO₂, Sc₂O₃, Y₂O₃, GeO₂, La₂O₃, CeO₂, PrO_(x)(x=appropriate integer), Nd₂O₃, Sm₂O₃, EuO_(y) (y=appropriate integer),Gd₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, SrTiO₃, BaTiO₃, PbTiO₃,PbZrO₃, Bi_(m)Ti_(n)O (m=appropriate integer; n=appropriate integer),Bi_(a)Si_(b)O (a=appropriate integer; b=appropriate integer), SrTa₂O₆,SrBi₂Ta₂O₉, YScO₃, LaAlO₃, NdAlO₃, GdScO₃, LaScO₃, LaLuO₃, Er₃Ga₅O₁₃.

Preferred metal nitrides may be selected from the group consisting ofBN, AlN, GaN, InN, Zr₃N₄, Cu₂N, Hf₃N₄, SiN_(c) (c=appropriate integer),TiN, Ta₃N₅, Ti—Si—N, Ti—AlN, TaN, NbN, MoN, WN_(d) (d=appropriateinteger), WN_(e)Cf (e=appropriate integer; f=appropriate integer).

The inorganic coating may comprise silica in any appropriate crystallineform.

The coating may incorporate an inorganic material in combination with anorganic or polymeric material. By way of example, in a preferredembodiment, the coating is an inorganic/polymer hybrid, such as asilica-acrylate hybrid material.

In a second preferred embodiment, the coating comprises a polymericmaterial, which may be a saturated or unsaturated hydrocarbon polymer,or may incorporate one or more heteroatoms (e.g. O, S, N, halogen) orheteroatom-containing functional groups (e.g. carbonyl, cyano, ether,epoxide, amide and the like).

Examples of preferred polymeric coating materials include acrylatepolymers (e.g. polymethyl(meth)acrylate, polybutylmethacrylate,polyoctylmethacrylate, alkylcyanoacryaltes, polyethyleneglycoldimethacrylate, polyvinylacetate etc), epoxides (e.g., EPOTEK 301 A+BThermal curing epoxy, EPOTEK OG112-4 single pot UV curing epoxy, orEX0135A and B Thermal curing epoxy), polyamides, polyimides, polyesters,polycarbonates, polythioethers, polyacrylonitryls, polydienes,polystyrene polybutadiene copolymers (Kratons), pyrelenes,poly-para-xylylene (parylenes), polyetheretherketone (PEEK),polyvinylidene fluoride (PVDF), polydivinyl benzene, polyethylene,polypropylene, polyethylene terephthalate (PET), polyisobutylene (butylrubber), polyisoprene, and cellulose derivatives (methyl cellulose,ethyl cellulose, hydroxypropylmethyl cellulose,hydroxypropylmethylcellulose phthalate, nitrocellulose), andcombinations thereof.

By incorporating quantum dots into primary particle materials of thekind described above and coating the particles it is possible to protectthe otherwise reactive quantum dots from the potentially damagingsurrounding chemical environment. Moreover, by placing a number ofquantum dots into a single bead, for example in the size range from 20nm to 500 μm in diameter, and providing the bead with a suitableprotective coating of, for example, a polymeric or inorganic material,the resulting coated QD-bead is more stable than either free “naked”quantum dots, or uncoated QD-beads to the types of chemical, mechanical,thermal and/or photo-processing steps which are required to incorporatequantum dots in most commercial applications, such as when employingquantum dots as down converters in a “QD-solid-state-LED” light emittingdevice.

Each primary particle may contain any desirable number and/or type ofsemiconductor nanoparticles. Thus, the primary matrix material of theprimary particle may contain a single type of semiconductornanoparticle, e.g. InP, InP/ZnS or CdSe, of a specific size range, suchthat the plurality of QD-containing beads emits monochromatic light of apre-defined wavelength, i.e. color. The color of the emitted light maybe adjusted by varying the type of semiconductor nanoparticle materialused, e.g. changing the size of the nanoparticle, the nanoparticle coresemiconductor material and/or adding one or more outer shells ofdifferent semiconductor materials.

Moreover, color control can also be achieved by incorporating differenttypes of semiconductor nanoparticles, for examples nanoparticles ofdifferent size and/or chemical composition within the primary matrixmaterial of each particle.

Furthermore, the color and color intensity can be controlled byselecting an appropriate number of semiconductor nanoparticles withineach particle. Preferably each primary particle contains at least around1000 semiconductor nanoparticles of one or more different types, morepreferably at least around 10,000, more preferably at least around50,000, and most preferably at least around 100,000 semiconductornanoparticles of one or more different types.

Where the primary particles are provided in the preferred form of beadsor microbeads, some or all of the beads preferably contain one or moresemiconductor nanoparticle capable of secondary light emission uponexcitation by primary light emitted by a primary light source (e.g. anLED).

The primary particles may be dispersed within an encapsulating medium,such as an LED encapsulant, to provide a robust QD-containingformulation which can then safely be used in subsequent processingsteps, for example, to deposit a desired amount of such a formulation onto an LED chip to provide a QD/LED-based light emitting device. Anydesirable number of beads may be dispersed or embedded within theencapsulating medium, for example, the formulation may contain 1 to10,000 beads, more preferably 1 to 5000 beads, and most preferably 5 to1000 beads.

It should also be appreciated that the encapsulating medium may haveembedded therein one or more type of semiconductornanoparticle-containing primary particles. That is, two or moredifferent types of primary particles (one or more containing thenanoparticles) may be embedded within the same encapsulating medium. Inthis way, where the population of nanoparticles contains more than onedifferent type of nanoparticle, the nature of the primary particle canbe selected for optimum compatibility with both the different types ofnanoparticles and the particular medium used.

Advantages of quantum dot-containing beads incorporatingstability-enhancing additives, optionally also incorporating a surfacecoating, over free quantum dots or uncoated quantum dot-containing beadscan include greater stability to air and moisture, greater stability tophoto-oxidation and/or greater stability to mechanical processing.Moreover, by pre-loading small microbeads, which can range in size froma few 50 nm to 500 μm, with quantum dots, adding a stability-enhancingadditive and optionally coating the individual microbeads prior toincorporating a plurality of such quantum dot-containing beads into anLED encapsulation material on a UV or blue LED, it is a relativelysimple process to change, in a controllable and reproducible manner, thecolor of the light emitted by the resulting LED-based light emittingdevice.

Semiconductor Nanoparticles

Any desirable type of semiconductor nanoparticle may be employed in thepresent invention. In a preferred embodiment, the nanoparticle containsions, which may be selected from any desirable group of the periodictable, such as but not limited to group 11, 12, 13, 14, 15 or 16 of theperiodic table. The nanoparticles may incorporate transition metal ionsor d-block metal ions. It is preferred that the nanoparticles containfirst and second ions with the first ion preferably selected from group11, 12, 13 or 14 and the second ion preferably selected from group 14,15 or 16 of the periodic table. The nanoparticles may contain one ormore semiconductor material selected from the group consisting of CdO,CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, InP, InAs, InSb, AIP, AIS, AlAs,AlSb, GaN, GaP, GaAs, GaSb, PbS, PbSe, Si, Ge, MgS, MgSe, MgTe andcombinations thereof. Moreover, the nanoparticles may be binary,tertiary or quaternary core, core-shell or core-multi shell, doped orgraded nanoparticles.

Any appropriate method may be employed to produce the semiconductornanoparticles employed in the various aspects of the present invention.That being said, it is preferred that said semiconductor nanoparticlesare produced by converting a nanoparticle precursor composition to thematerial of the nanoparticles in the presence of a molecular clustercompound under conditions permitting seeding and growth of thenanoparticles on the cluster compound. The method may employ themethodology set out in the applicant's co-pending European patentapplication (publication no. EP1743054A).

Conveniently, the nanoparticles incorporate first and second ions andthe nanoparticle precursor composition comprises first and secondnanoparticle precursor species containing the first and second ionsrespectively which are combined, preferably in the presence of amolecular cluster compound, as exemplified below in Synthetic Methods1.1 and 1.2.

The first and second precursor species may be separate species in theprecursor composition or may form part of a single molecular speciescontaining both the first and second ions.

In the preferred embodiments employing a molecular cluster compound, itis preferred that the molecular clusters contain third and fourth ions.At least one of said third and fourth ions is preferably different tosaid first and second ions contained in the first and secondnanoparticle precursor species respectively. The third and fourth ionsmay be selected from any desirable group of the periodic table, such asbut not limited to group 11, 12, 13, 14, 15 or 16 of the periodic table.The third and/or fourth ion may be a transition metal ion or a d-blockmetal ion. Preferably the third ion is selected from group 11, 12, 13 or14 and the fourth ion is selected from group 14, 15 or 16 of theperiodic table.

By way of example, the molecular cluster compound may incorporate thirdand fourth ions from groups 12 and 16 of the periodic table respectivelyand the first and second ions derived from the first and secondnanoparticle precursor species may be taken from groups 13 and 15 of theperiodic table respectively as in Synthetic Method 1.2. The methodologydescribed in the Applicant's co-pending International PCT PatentApplication (Application No. PCT/GB2008/002560) may be employed.

It will be appreciated that during the reaction of the first and secondnanoparticle precursor species, the first nanoparticle precursor speciesmay be added in one or more portions and the second nanoparticleprecursor species may be added in one or more portions. The firstnanoparticle precursor species is preferably added in two or moreportions. In this case, it is preferred that the temperature of areaction mixture containing the first and second nanoparticle precursorspecies is increased between the addition of each portion of the firstprecursor species. Additionally or alternatively, the secondnanoparticle precursor species may be added in two or more portions,whereupon the temperature of a reaction mixture containing the first andsecond nanoparticle precursor species may be increased between theaddition of each portion of the second precursor species. Themethodology described in the applicant's co-pending European patentapplication (Application No. 06808360.9) may be used.

The coordination about the final inorganic surface atoms in any core,core-shell or core-multi shell, doped or graded nanoparticle istypically incomplete, with highly reactive non-fully coordinated atomsacting as “dangling bonds” on the surface of the particle, which canlead to particle agglomeration. This problem is typically overcome bypassivating (capping) the “bare” surface atoms with protecting organicgroups.

In many cases, the capping agent is the solvent in which thenanoparticles have been prepared, and consists of a Lewis base compound,or a Lewis base compound diluted in an inert solvent such as ahydrocarbon. There is a lone pair of electrons on the Lewis base cappingagent that are capable of a donor type coordination to the surface ofthe nanoparticle; ligands of this kind include, but are not limited to,mono- or multi-dentate ligands such as phosphines (trioctylphosphine,triphenylphosphine, t-butylphosphine etc.), phosphine oxides(trioctylphosphine oxide, triphenylphosphine oxide etc.), alkylphosphonic acids, alkyl-amines (hexadecylamine, octylamine etc.),aryl-amines, pyridines, long chain fatty acids and thiophenes.

In addition to the outermost layer of organic material or sheathmaterial (capping agent) helping to inhibit nanoparticle-nanoparticleaggregation, this layer can also protect the nanoparticles from theirsurrounding electronic and chemical environments, and provide a means ofchemical linkage to other inorganic, biological or organic material,whereby the functional group is pointing away from the nanoparticlesurface and is available to bond/react/interact with other availablemolecules. Examples include, but are not limited to, amines, alcohols,carboxylic acids, esters, acid chloride, anhydrides, ethers, alkylhalides, amides, alkenes, alkanes, alkynes, allenes, amino acids,azides, groups etc. The outermost layer (capping agent) of a quantum dotcan also consist of a coordinated ligand that processes a functionalgroup that is polymerizable and can be used to form a polymer layeraround the nanoparticle. The outermost layer can also consist of organicunits that are directly bonded to the outermost inorganic layer such asvia a disulphide bond between the inorganic surface (e.g. ZnS) and athiol capping molecule. These can also possess additional functionalgroup(s), not bonded to the surface of the particle, which can be usedto form a polymer around the particle, or for furtherreaction/interaction/chemical linkage.

An example of a material to which nanoparticle surface binding ligandscan be linked is the primary matrix material from which the primaryparticles are formed. There are a number of approaches to incorporatesemiconductor nanoparticles, such as quantum dots, into the types ofprimary matrix materials described hereinbefore by pre-coating thenanoparticles with ligands that are compatible in some way with thematrix material of the primary particles. By way of example, in thepreferred embodiment where the semiconductor nanoparticles are to beincorporated into polymeric beads, the nanoparticles can be produced soas to possess surface ligands which are polymerizable, hydrophobic,hydrophilic, positively or negatively charged, or functionalized with areactive group capable of associating with the polymer of the polymericbeads by chemical reaction, covalent linkage or non-covalent interaction(e.g. interchelation).

The inventors have determined that it is possible to take quantum dotsprepared using any desirable method, incorporate these quantum dots intosilica or polymer beads also including at least one type ofstability-enhancing additive, and then optionally coat the beads with aprotective barrier layer of a material such as a polyacrylate ordielectric metal oxide like aluminum oxide, to provide significantlymore robust, easily processable quantum dot-containing materials.Quantum dot-containing beads of this kind can be employed in a widerange of applications, particularly, but not exclusively, thefabrication of LED-based light emitting devices wherein the QD-beads areembedded within a host LED encapsulant and then deposited onto asolid-state LED chip to form a quantum dot-based light emitting device.

Incorporating Quantum Dots into Beads

Considering the initial step of incorporating quantum dots into beads, afirst option is to incorporate the quantum dots directly into the beads.A second option is to immobilize the quantum dots in beads throughphysical entrapment. It is possible using these methods to make apopulation of beads that contain just a single type of quantum dot (e.g.one color) by incorporating a single type of quantum dot into the beads.Alternatively, it is possible to construct beads that contain 2 or moretypes of quantum dot (e.g. two or more colors) by incorporating amixture of two or more types of quantum dot (e.g. material and/or size)into the beads. Such mixed beads can then be combined in any suitableratio to emit any desirable color of secondary light followingexcitation by the primary light emitted by the primary light source(e.g. LED). This is exemplified in FIGS. 4 to 6 below whichschematically show QD-bead light emitting devices includingrespectively: a) multi-colored, multiple quantum dot types in each beadsuch that each bead emits white secondary light; b) multi-colored,multiple quantum dot types in different beads such that each beadcontains a single quantum dot type emitting a single color, a mixture ofthe beads combining to produce white secondary light; and c) singlycolored, single quantum dot type in all beads such that a mixture of thebeads emits a single color of secondary light, e.g. red.

The or each stability-enhancing additive may be added to the quantumdot-containing beads during initial bead formation and/or after thebeads have been formed independently of which of the two options set outabove are employed to incorporate the quantum dots within the beads.

Incorporating Quantum Dots Beads During Bead Formation

With regard to the first option of incorporating the quantum dotsdirectly into the primary particles (i.e. the beads) during beadformation, by way of example, suitable core, core/shell orcore/multishell nanoparticles (e.g. InP/ZnS core/shell quantum dots) maybe contacted by one or more bead precursors (e.g. an acrylate monomer, asilicate material, or a combination of both) and then subjected tosuitable conditions (e.g. introduction of a polymerization initiator) toform the bead material. One or more stability-enhancing additive may beincluded in the reaction mixture in which the nanoparticles arecontacted by the bead precursors. Moreover, at this stage, a surfacecoating can be applied to the beads.

By way of further example, hexadecylamine-capped CdSe-basedsemiconductor nanoparticles can be treated with at least one, morepreferably two or more polymerizable ligands (optionally one ligand inexcess) resulting in the displacement of at least some of thehexadecylamine capping layer with the polymerizable ligand(s). Thedisplacement of the capping layer with the polymerizable ligand(s) canbe accomplished by selecting a polymerizable ligand or ligands withstructures similar to that of trioctylphosphine oxide (TOPO), which is aligand with a known and very high affinity for CdSe-based nanoparticles.It will be appreciated that this basic methodology may be applied toother nanoparticle/ligand pairs to achieve a similar effect. That is,for any particular type of nanoparticle (material and/or size), it ispossible to select one or more appropriate polymerizable surface bindingligands by choosing polymerizable ligands comprising a structural motifwhich is analogous in some way (e.g. has a similar physical and/orchemical structure) to the structure of a known surface binding ligand.Once the nanoparticles have been surface-modified in this way, they canthen be added to a monomer component of a number of micro-scalepolymerization reactions to form a variety of quantum dot-containingresins and beads.

Examples of polymerization methods that may be used to construct quantumdot-containing beads include, but are not limited to, suspension,dispersion, emulsion, living, anionic, cationic, RAFT, ATRP, bulk, ringclosing metathesis and ring opening metathesis. Initiation of thepolymerization reaction may be caused by any appropriate means thatcauses the monomers to react with one another, such as free radicals,light, ultrasound, cations, anions, heat.

A preferred method is suspension polymerization involving thermal curingof one or more polymerizable monomers from which the primary matrixmaterial is to be formed. Said polymerizable monomers may, for example,comprise methyl (meth)acrylate, ethylene glycol dimethacrylate and/orvinyl acetate.

Quantum dot-containing beads may be generated simply by adding quantumdots to the mixture of reagents used to construct the beads. In someinstances quantum dots (nascent quantum dots) will be used as isolatedfrom the reaction employed to synthesize them and are thus generallycoated with an inert outer organic ligand layer. In an alternativeprocedure a ligand exchange process may be carried out prior to the beadforming reaction. Here one or more chemically reactive ligands (forexample this might be a ligand for the quantum dots which also containsa polymerizable moiety) is added in excess to a solution of nascentquantum dots coated in an inert outer organic layer. After anappropriate incubation time the quantum dots are isolated, for exampleby precipitation and subsequent centrifugation, washed and thenincorporated into the mixture of reagents used in the bead formingreaction/process.

Both quantum dot incorporation strategies will result in statisticallyrandom incorporation of the quantum dots into the beads and thus thepolymerization reaction will result in beads containing statisticallysimilar amounts of the quantum dots and, optionally, the one or moreadditives. Bead size can be controlled by the choice of polymerizationreaction used to construct the beads, and additionally, once apolymerization method has been selected, bead size can also becontrolled by selecting appropriate reaction conditions, e.g. in asuspension polymerization reaction by stirring the reaction mixture morequickly to generate smaller beads. Moreover the shape of the beads canbe readily controlled by choice of procedure in conjunction with whetheror not the reaction is carried out in a mold. The composition of thebeads can be altered by changing the composition of the monomer mixturefrom which the beads are constructed. Similarly the beads can also becross-linked with varying amounts of one or more crosslinking agents(e.g. divinyl benzene). If beads are constructed with a high degree ofcrosslinking, e.g. greater than 5 mol % cross-linker, it may bedesirable to incorporate a porogen (e.g. toluene or cyclohexane) duringthe reaction used to construct the beads. The use of a porogen in such away leaves permanent pores within the matrix constituting each bead.These pores may be sufficiently large to allow the ingress of quantumdots into the bead.

Incorporating Quantum Dots into Prefabricated Beads

In respect of the second option for incorporating quantum dots into theprimary particles, the quantum dots can be immobilized within theprimary matrix material through physical entrapment. For example, asolution of quantum dots in a suitable solvent (e.g. an organic solvent)can be incubated with a sample of primary particles. Removal of thesolvent using any appropriate method results in the quantum dotsbecoming immobilized within the primary matrix material of the primaryparticles. The quantum dots remain immobilized in the particles unlessthe sample is resuspended in a solvent (e.g. organic solvent) in whichthe quantum dots are freely soluble. One or more stability-enhancingadditives may, for example, be included in the quantum dot solutionwhich is incubated with the primary particles. Alternatively, thequantum dots may first be added to the primary particles, and the one ormore additives then added to the primary particles. Additionally, atthis stage, a surface coating can be applied to the primary particles ifdesired.

In a further preferred embodiment, at least a portion of thesemiconductor nanoparticles can be physically attached to theprefabricated primary particles. Attachment may be achieved byimmobilization of a portion of the semiconductor nanoparticles withinthe polymer matrix of the prefabricated primary particles or bychemical, covalent, ionic, or physical connection between thesemiconductor nanoparticles and the prefabricated primary particles. Ina particularly preferred embodiment the prefabricated primary particlescomprise polystyrene, polydivinyl benzene and a polythiol.

Quantum dots can be irreversibly incorporated into prefabricated primaryparticles in a number of ways, e.g. chemical, covalent, ionic, physical(e.g. by entrapment) or any other form of interaction. If prefabricatedprimary particles are to be used for the incorporation of quantum dots,the solvent accessible surfaces of the primary particles may bechemically inert (e.g. polystyrene) or alternatively they may bechemically reactive/functionalized (e.g. Merrifield's Resin). Thechemical functionality may be introduced during the construction of theprimary particles, for example by the incorporation of a chemicallyfunctionalized monomer, or alternatively, chemical functionality may beintroduced in a post-particle construction treatment step, for exampleby conducting a chloromethylation reaction. Additionally chemicalfunctionality may be introduced by a post-particle construction stepinvolving a polymeric graft or other similar process whereby chemicallyreactive polymer(s) are attached to the outer layers/accessible surfacesof the bead. More than one such post construction derivatization processmay be carried out to introduce chemical functionality onto/into theprimary particles.

As with quantum dot incorporation into primary particles during theparticle forming reaction (i.e. the first option described above) thepre-fabricated primary particles can be of any shape, size andcomposition and may have any degree of cross-linker and may containpermanent pores if constructed in the presence of a porogen. Quantumdots may be imbibed into the primary particles by incubating a solutionof quantum dots in an organic solvent and adding this solvent to theprimary particles. The solvent must be capable of wetting the primaryparticles, and in the case of lightly crosslinked primary particles,preferably 0-10% crosslinked and most preferably 0-2% crosslinked, thesolvent should cause the polymer matrix to swell in addition tosolvating the quantum dots. Once the quantum dot-containing solvent hasbeen incubated with the primary particles it can be removed, for exampleby heating the mixture and causing the solvent to evaporate and thequantum dots to become embedded in the primary matrix materialconstituting the primary particles, or alternatively, by the addition ofa second solvent in which the quantum dots are not readily soluble butwhich mixes with the first solvent causing the quantum dots toprecipitate within the primary matrix material. Immobilization may bereversible if the primary particles are not chemically reactive, or elseif the primary particles are chemically reactive, the quantum dots maybe held permanently within the primary matrix material, by chemical,covalent, ionic, or any other form of interaction. Any desirablestability-enhancing additive can be added during any of the stages ofthe quantum dot—bead fabrication described above.

Incorporation of Quantum Dots into Sol-Gels to Produce Glass

As stated above, a preferred primary matrix material is an opticallytransparent media, such as a sol-gel or a glass. Such primary matrixmaterials may be formed in an analogous fashion to the method used toincorporate quantum dots into primary particles during the particleforming process as described above. For example, a single type ofquantum dot (e.g. one color) may be added to a reaction mixture used toproduce a sol-gel or glass. Alternatively, two or more types of quantumdot (e.g. two or more colors) may be added to a reaction mixture used toproduce a sol-gel or glass. The sol-gels and glasses produced by theseprocedures may have any shape, morphology or 3-dimensional structure.For example, the resulting primary particles may be spherical,disc-like, rod-like, ovoid, cubic, rectangular, or any of many otherpossible configurations. Any of the stability-enhancing additivesdescribed hereinbefore may be added to quantum dot-containing glassbeads. Some silica-based beads exhibit relatively low porosity ascompared, for example, to polymeric resin beads (e.g. acrylate-basedbeads). It may therefore be advantageous to add the or each additiveduring initial bead formation when the beads are made from asilica-based material rather than adding the additive(s) after beadformation, which may be more advantageous or desirable when using moreporous bead materials.

Application of Optional Surface Coating

In a preferred embodiment, where it is desired to provide a surfacecoating comprising an inorganic material on the quantum dot-containingprimary particles, such as a metal oxide or metal nitride, aparticularly preferred process to deposit the coating is atomic layerdeposition (ALD), although it will be appreciated that other suitabletechniques can be employed.

The provision of a surface coating by ALD, using a metal oxide surfacecoating as an example, comprises the following four basic steps:

1) Exposing a surface of a quantum dot-containing primary particle to ametal precursor;2) Purging the reaction chamber containing the primary particles toremove non-reacted metal precursor and any gaseous reaction by-products;3) Exposing the surface of the primary particles to an oxide precursor;and4) Purging the reaction chamber.

The above steps can then be repeated any desirable number of times toprovide a surface coating of the desired thickness, for example, athickness of around 1 to 500 nm. Each reaction cycle adds apredetermined amount of coating material to the surface of the primaryparticles. One cycle may take time from around 0.5 seconds to around 2-3seconds and deposit between 1 and 30 nm of surface coating.

Before initiating the ALD process, it is preferred that the surface ofthe primary particles is heat treated to ensure their stability duringthe ALD process. It will be appreciated that since ALD is essentially asurface-controlled process, where process parameters other than theprecursors, substrate (i.e. primary particle material), reactiontemperature (typically around 100 to 400° C., but can be as high as 500°C.), and, to a lesser extent pressure (typically around 1 to 10 mbar),have little or no influence on the final surface coating, ALD-grownsurface layers or films are extremely conformal and uniform inthickness, making ALD is a particularly preferred method for depositingprotective coatings on to the surface of quantum dot-containing primaryparticles.

A particularly preferred surface coating is Al₂O₃. An Al₂O₃ surfacecoating of only up to around 20 to 30 nm applied by ALD at a temperatureof around 100 to 175° C. using trimethylaluminum and water as precursorscan exhibit a very low water vapor transmission rate and permeability toother gases and liquids.

In an alternative preferred embodiment, the surface coating may beproduced in-situ on the surface of the primary particles. By way ofexample, a surface of quantum dot-containing primary particles can becontacted by polymerizable monomers, which are then polymerized on thesurface of the particles to produce a polymeric surface coating on theparticles. One method by which contacting of the particles by themonomers may be effected is to disperse the particles within a monomermixture, optionally including a crosslinking agent and, if necessary, apolymerization initiator, such as a photoinitiator. Polymerization maythen be effected in any manner appropriate for the monomers being used,for example if photopolymerizable monomers are used, then the polymermixture containing the primary particles and the optional photoinitiatormay then be exposed to a suitable source of radiation (e.g. UV).

FIGS. 11 to 14 depict alternative preferred arrangements of quantumdot-containing primary particles provided directly or indirectly with aprotective surface coating.

FIG. 11 illustrates a population of quantum dots entrapped within aprimary particle in the form of a polymer bead according to a preferredembodiment of the present invention. The primary particle is providedwith a surface coating of an inorganic material, before being dispersedwithin a secondary matrix material in the form of an LED encapsulantdisposed on an LED to provide a light emitting device according to apreferred embodiment of the present invention. FIG. 12 depicts apopulation of quantum dots entrapped within a primary particle in theform of a polymer bead made from a first type of polymer (polymer 1)which is encapsulated within a second type of polymer material (polymer2). The surface of the second type of polymer is provided with aprotective surface coating of an inorganic material according to apreferred embodiment of the present invention. The encapsulated primaryparticles are then dispersed within a secondary matrix material in theform of an LED encapsulant disposed on an LED to provide a lightemitting device according to a preferred embodiment of the presentinvention. FIG. 13 illustrates a population of quantum dots entrappedwithin a population of primary particles in the form of polymer beads(bead 1) according to a preferred embodiment of the present invention inwhich each of the primary particles is provided with a surface coatingof an inorganic material. The coated primary particles are showndispersed within a second type of bead (bead 2) to produce a“bead-in-bead” composite material, which can be dispersed, as shown,within a secondary matrix material in the form of an LED encapsulantdisposed on an LED to provide a light emitting device according to apreferred embodiment of the present invention. FIG. 14 depicts apopulation of quantum dots entrapped within a population of primaryparticles in the form of polymer beads according to a preferredembodiment of the present invention, the population of primary particlesbeing dispersing within a second type of bead to produce a“bead-in-bead” composite material which is then provided with aninorganic surface coating layer. The coated bead-in-bead compositematerial can then be dispersed within a secondary matrix material asshown in the form of an LED encapsulant disposed on an LED to provide alight emitting device according to a preferred embodiment of the presentinvention.

Application of QD-Beads—Incorporation into LED Encapsulant

While the addition of one or more additives to beads containing quantumdots has many advantages as outlined above, one significant advantage ofthe present invention is that additive-containing quantum dot-beads(QD-beads) produced as described above can be incorporated intocommercially available LED encapsulant materials simply by weighing thedesired amount of the QD-bead material and adding this to the desiredamount of LED encapsulant material.

It is preferred that the bead/encapsulant composite is mixed thoroughlyto provide a homogeneous mixture. The mixture may then be dispensed ontoa commercially available LED and cured according to normal curingprocedures for the particular LED-encapsulant used. Theadditive-containing QD-beads thus provide a simple and straightforwardway of facilitating the formulation of bead/LED encapsulant compositeswhich can be used in the fabrication of next generation, higherperformance light emitting devices using, as far as possible, standardcommercially available materials and methods.

LED Encapsulating Materials

Any existing commercially available LED encapsulant may be used withadditive-containing QD-beads produced according to the presentinvention. Preferred LED encapsulants include silicones, epoxies,(meth)acrylates and other polymers, although it will be appreciated bythe skilled person that further options are available, such as but notlimited to silica glass, silica gel, siloxane, sol gel, hydrogel,agarose, cellulose, epoxy, polyether, polyethylene, polyvinyl,poly-diacetylene, polyphenylene-vinylene, polystyrene, polypyrrole,polyimide, polyimidazole, polysulfone, polythiophene, polyphosphate,poly(meth)acrylate, polyacrylamide, polypeptide, polysaccharide andcombinations thereof.

LED encapsulants which may be used comprise, but are not limited to, UVcurable encapsulants and heat curable encapsulants, includingencapsulants which require one or more catalysts to support the curingprocess. Specific examples of commercially available siliconeencapsulants, which are suitable may be selected from the groupconsisting of SCR1011, SCR1012, SCR1016. LPS-3412 (all available fromShin Etsu) and examples of suitable epoxy encapsulents may be selectedfrom the group consisting of Pacific Polytech PT1002, Fine PolymersEpifine EX-1035A, and Fine Polymers Epifine X-1987.

Color Indexing

The color of the light output from an additive-containing QD-bead-LED(the “secondary light”) can be measured using a spectrometer. Thespectral output (mW/nm) can then be processed mathematically so that theparticular color of the light emitting device can be expressed as colorcoordinates on a chromaticity diagram, for example the 2° CIE 1931chromaticity diagram (see FIG. 2).

The 2° CIE 1931 chromaticity coordinates for a particular spectrum canbe calculated from the spectral power distribution and the CIE 1931color matching functions x, y, z (see FIG. 3). The correspondingtristimulus values can be calculated thus

X=∫pxdλ Y=∫pydλ Z=∫pzdλ,

Where p is the spectral power, and x, y and z are the color matchingfunctions. From X, Y, and Z the chromaticity coordinates x, y can becalculated according to

${x = \frac{X}{X + Y + Z}}\mspace{14mu}$ and $y = \frac{Y}{X + Y + Z}$

Using x, y as the coordinates, a two-dimensional chromaticity diagram(the CIE 1931 color space diagram) can be plotted which is analogous tothe exemplary diagram depicted in FIG. 2.

Color Rendering

Color rendering describes the ability of a light source to illuminateobjects such that they appear the correct color when compared to howthey appear when illuminated by a reference light source. Usually thereference light source is a tungsten filament bulb, which is assigned acolor rendering index (CRI) of 100. To be acceptable for generallighting, a white light emitting device source is required to have aCRI>80. An example of poor color rendering is the sodium street lampwhich has very poor color rendering capability, i.e. it is difficult todistinguish a red car from a yellow car illuminated by a sodium lamp, inthe dark under a sodium lamp they both appear grey.

The present invention provides a plurality of robust, high performanceadditive-containing QD-beads which can be used to fabricate alight-emitting device. The quantum dots within the primary particles orbeads are in optical communication with a primary solid-statephoton/light source (e.g. an LED, laser, arc lamp or black-body lightsource) such that, upon excitation by primary light from the primarylight source the quantum dots within the primary particles emitsecondary light of a desired color. The required intensities andemission wavelengths of the light emitted from the device itself can beselected according to appropriate mixing of the color of the primarylight with that of the secondary light(s) produced from the downconversion of the primary light by the quantum dots. Moreover, the size(and thus emission) and number of each type of quantum dot within theprimary particles can be controlled, as can the size, morphology andconstituency of the primary matrix material making up the primaryparticles, such that subsequent mixing of the quantum dot-containingmedia allows light of any particular color and intensity to be produced.

It will be appreciated that the overall light emitted from the devicemay consist effectively of the light emitted from the quantum dots, i.e.just the secondary light, or a mixture of light emitted from the quantumdots and light emitted from the solid-state/primary light source, i.e. amixture of the primary and secondary light. Color mixing of the quantumdots can be achieved either within the quantum dot-containing media(e.g. within each bead in a population of beads such that each beadcontains a number of different size/color emitting quantum dots) or amixture of differently colored primary matrix materials with all thequantum dots within a specific matrix material being the same size/color(e.g. some beads containing all green quantum dots and others containingall red quantum dots).

The present invention is illustrated with reference to the followingnon-limiting examples and figures in which:

BRIEF DESCRIPTION OF FIGURES

FIG. 1 schematically depicts a quantum dot-based light emitting deviceaccording to an aspect of the present invention;

FIG. 2 is a 2° CIE 1931 chromaticity diagram;

FIG. 3 is a 2° CIE 1931 color matching diagram matching functions x, y,z;

FIG. 4 is a schematic representation of an additive-containingQD-bead-based light emitting device employing multi-colored, multiplequantum dot types in each bead such that each bead emits white secondarylight;

FIG. 5 is a schematic representation of an additive-containingQD-bead-based light emitting device employing multi-colored, multiplequantum dot types in different beads such that each bead contains asingle quantum dot type emitting a single color, a mixture of the beadscombining to produce white secondary light;

FIG. 6 is a schematic representation of an additive-containingQD-bead-based light emitting device employing a singly colored, singlequantum dot type in all beads such that a mixture of the beads emits asingle color of secondary light (in this case, red light);

FIG. 7 is a schematic representation of a process according to a firstembodiment of the present invention wherein an additive is combined witha population of quantum dots during formation of a primary particlecontaining the quantum dots and, consequently, the additive;

FIG. 8 is a schematic representation of a process according to a secondembodiment of the present invention wherein quantum dots are firstencapsulated within a bead formed of a first type of polymer (polymer 1)and then an additive is combined with the quantum dot-containing beadduring formation of a primary particle made from a second type ofpolymer (polymer 2) containing the quantum dot-containing bead and,consequently, the additive;

FIG. 9 is a schematic representation of a process according to a thirdembodiment of the present invention wherein quantum dots are firstencapsulated within a population of beads formed of a first type ofpolymer (polymer 1) and then an additive is combined with the quantumdot-containing beads during formation of a primary particle made from asecond type of polymer (polymer 2) containing the quantum dot-containingbeads and, consequently, the additive;

FIG. 10 is a schematic representation of a process according to a fourthembodiment of the present invention wherein quantum dots are firstencapsulated within a population of beads formed of a first type ofpolymer (polymer 1), which are then encapsulated within a bead formed ofa second type of polymer (polymer 2), and then an additive is combinedwith the quantum dot-containing beads during formation of a primaryparticle made from a third type of polymer (polymer 3) containing thequantum dot-containing beads and, consequently, the additive;

FIG. 11 is a schematic representation of a population of quantum dotsentrapped within a primary particle in the form of a polymer beadaccording to a preferred embodiment of the present invention in whichthe primary particle is provided with a surface coating of an inorganicmaterial, and the primary particles are dispersed within a secondarymatrix material in the form of an LED encapsulant disposed on an LED toprovide a light emitting device according to a preferred embodiment ofthe present invention;

FIG. 12 is a schematic representation of a population of quantum dotsentrapped within a primary particle in the form of a polymer bead madefrom a first type of polymer (polymer 1) which is encapsulated within asecond type of polymer material (polymer 2) which is provided with asurface coating of an inorganic material according to a preferredembodiment of the present invention, and the encapsulated primaryparticles are dispersed within a secondary matrix material in the formof an LED encapsulant disposed on an LED to provide a light emittingdevice according to a preferred embodiment of the present invention;

FIG. 13 is a schematic representation of a population of quantum dotsentrapped within a population of primary particles in the form ofpolymer beads (bead 1) according to a preferred embodiment of thepresent invention in which each of the primary particles is providedwith a surface coating of an inorganic material, before dispersing thecoated primary particles within a second type of bead (bead 2) toproduce a “bead-in-bead” composite material, and then dispersing thebead-in-bead composite material within a secondary matrix material inthe form of an LED encapsulant disposed on an LED to provide a lightemitting device according to a preferred embodiment of the presentinvention; and

FIG. 14 is a schematic representation of a population of quantum dotsentrapped within a population of primary particles in the form ofpolymer beads according to a preferred embodiment of the presentinvention, the population of primary particles being dispersing within asecond type of bead to produce a “bead-in-bead” composite material whichis then provided with an inorganic surface coating layer, and thendispersing the bead-in-bead composite material within a secondary matrixmaterial in the form of an LED encapsulant disposed on an LED to providea light emitting device according to a preferred embodiment of thepresent invention.

EXAMPLES

Examples 1 and 2 below describe the preparation of additive-containingquantum dot beads, and Example 3 describes how to coat such beads, whichcould, for example, be in used in the fabrication of new, improvedquantum dot-based light emitting devices. The Synthetic Methods sectionprovides two methods for producing quantum dots (1.1 and 1.2) and threemethods for incorporating quantum dots into primary particles or “beads”(2.1, 2.2 and 2.3).

Synthetic Methods 1.1 Preparation of CdSe/ZnS Core/Shell Quantum DotsPreparation of CdSe Cores

HDA (500 g) was placed in a three-neck round bottomed flask and driedand degassed by heating to 120° C. under a dynamic vacuum for >1 hour.The solution was then cooled to 60° C. To this was added 0.718 g of[HNEt₃]₄[Cd₁₀Se₄(SPh)₁₆] (0.20 mmols). In total 42 mmols, 22.0 ml ofTOPSe and 42 mmols, (19.5 ml, 2.15 M) of Me₂Cd.TOP was used. Initially 4mmol of TOPSe and 4 mmols of Me₂Cd.TOP were added to the reaction atroom temperature and the temperature increased to 110° C. and allowed tostir for 2 hours. The reaction was a deep yellow color, the temperaturewas progressively increased at a rate of ˜1° C./5 min with equimolaramounts of TOPSe and Me₂Cd.TOP being added dropwise. The reaction wasstopped when the PL emission maximum had reached ˜600 nm, by cooling to60° C. followed by addition of 300 ml of dry ethanol or acetone. Thisproduced a precipitation of deep red particles, which were furtherisolated by filtration. The resulting CdSe particles were recrystallizedby re-dissolving in toluene followed by filtering through Celitefollowed by re-precipitation from warm ethanol to remove any excess HDA,selenium or cadmium present. This produced 10.10 g of HDA capped CdSenanoparticles. Elemental analysis C=20.88, H=3.58, N=1.29, Cd=46.43%.Max PL=585 nm, FWHM=35 nm. 38.98 mmols, 93% of Me₂Cd consumed in formingthe quantum dots.

Growth of ZnS Shell

HDA (800 g) was placed in a three neck round-bottom flask, dried anddegassed by heating to 120° C. under a dynamic vacuum for >1 hour. Thesolution was then cooled to 60° C., to this was added 9.23 g of CdSenanoparticles that have a PL maximum emission of 585 nm. The HDA wasthen heated to 220° C. To this was added by alternate dropwise additiona total of 20 ml of 0.5 M Me₂Zn.TOP and 0.5 M, 20 ml of sulfur dissolvedin octylamine. Three alternate additions of 3.5, 5.5 and 11.0 ml of eachwere made, whereby initially 3.5 ml of sulfur was added dropwise untilthe intensity of the PL maximum was near zero. Then 3.5 ml of Me₂Zn.TOPwas added dropwise until the intensity of the PL maximum had reached amaximum. This cycle was repeated with the PL maximum reaching a higherintensity with each cycle. On the last cycle, additional precursor wasadded once the PL maximum intensity been reached until it was between5-10% below the maximum intensity, and the reaction was allowed toanneal at 150° C. for 1 hour. The reaction mixture was then allowed tocool to 60° C. whereupon 300 ml of dry “warm” ethanol was added whichresulted in the precipitation of particles. The resulting CdSe—ZnSparticles were dried before re-dissolving in toluene and filteringthrough Celite followed by re-precipitation from warm ethanol to removeany excess HDA. This produced 12.08 g of HDA capped CdSe—ZnS core-shellnanoparticles. Elemental analysis C=20.27, H=3.37, N=1.25, Cd=40.11,Zn=4.43%; Max PL 590 nm, FWHM 36 nm.

1.2 Preparation of InP/ZnS Core/Shell Quantum Dots Preparation of MPCores (500-700 nm Emission)

Di-butyl ester (100 ml) and Myristic acid (10.0627 g) were placed in athree-neck flask and degassed at 70° C. under vacuum for one hour. Afterthis period, nitrogen was introduced and the temperature increased to90° C. ZnS molecular cluster [Et₃NH]₄[Zn₁₀S₄(SPh)₁₆] (4.7076 g) wasadded and the mixture allowed to stir for 45 minutes. The temperaturewas then increased to 100° C. followed by the dropwise addition ofIn(MA)₃ (1 M, 15 ml) followed by (TMS)₃P (1 M, 15 ml). The reactionmixture was allowed to stir while increasing the temperature to 140° C.At 140° C., further dropwise additions of In(MA)₃ (1 M, 35 ml) (left tostir for 5 minutes) and (TMS)₃P (1 M, 35 ml) were made. The temperaturewas then slowly increased to 180° C. and further dropwise additions ofIn(MA)₃ (1 M, 55 ml) followed by (TMS)₃P (1 M, 40 ml) were made. Byaddition of the precursor in the manner above nanoparticles of InP couldbe grown with the emission maximum gradually increasing from 520 nm upto 700 nm, whereby the reaction can be stopped when the desired emissionmaximum has been obtained and left to stir at this temperature for halfan hour. After this period, the temperature was decreased to 160° C. andthe reaction mixture was left to anneal for up to 4 days (at atemperature between 20-40° C. below that of the reaction). A UV lamp wasalso used at this stage to aid in annealing.

The nanoparticles were isolated by the addition of dried degassedmethanol (approx. 200 ml) via cannula techniques. The precipitate wasallowed to settle and then methanol was removed via cannula with the aidof a filter stick. Dried degassed chloroform (approx. 10 ml) was addedto wash the solid. The solid was left to dry under vacuum for 1 day.This produced 5.60 g of InP core nanoparticles. Elemental analysis: maxPL=630 nm, FWHM=70 nm.

Post-Operative Treatments

The quantum yields of the InP quantum dots prepared above were increasedby washing with dilute HF acid. The dots were dissolved in anhydrousdegassed chloroform (˜270 ml). A 50 ml portion was removed and placed ina plastic flask, flushed with nitrogen. Using a plastic syringe, the HFsolution was made up by adding 3 ml of 60% w/w HF in water and adding todegassed THF (17 ml). The HF was added dropwise over 5 hrs to the InPdots. After addition was complete the solution was left to stirovernight. Excess HF was removed by extracting through calcium chloridesolution in water and drying the etched InP dots. The dried dots werere-dispersed in 50 ml chloroform for future use. Max 567 nm, FWHM 60 nm.The quantum efficiencies of the core materials at this stage range from25-90%

Growth of ZnS Shell

A 20 ml portion of the HF etched InP core particles was dried down in a3-neck flask. 1.3 g myristic acid and 20 ml di-n-butyl sebacate esterwas added and degassed for 30 minutes. The solution was heated to 200°C. then 1.2 g anhydrous zinc acetate was added and 2 ml 1 M (TMS)₂S wasadded dropwise (at a rate of 7.93 ml/hr) after addition was complete thesolution was left to stir. The solution was kept at 200° C. for 1 hrthen cooled to room temperature. The particles were isolated by adding40 ml of anhydrous degassed methanol and centrifuged. The supernatantliquid was disposed of and to the remaining solid 30 ml of anhydrousdegassed hexane was added. The solution was allowed to settle for 5 hrsand then re-centrifuged. The supernatant liquid was collected and theremaining solid was discarded. PL emission peak Max.=535 nm, FWHM=65 nm.The quantum efficiencies of the nanoparticle core/shell materials atthis stage ranged from 35-90%.

2.1 Incorporating Quantum Dots into Suspension Polymeric Beads

1% wt/vol polyvinyl acetate (PVA) (aq) solution was prepared by stirringfor 12 hours followed by extensive degassing by bubbling nitrogenthrough the solution for a minimum of 1 hour. The monomers, methylmethacrylate and ethylene glycol dimethacrylate, were also degassed bynitrogen bubbling and used with no further purification. The initiatorAIBN (0.012 g) was placed into the reaction vessel and put under threevacuum/nitrogen cycles to ensure no oxygen was present.

CdSe/ZnS core/shell quantum dots as prepared above in Method 1 wereadded to the reaction vessel as a solution in toluene and the solventremoved under reduced pressure. Degassed methyl methacrylate (0.98 mL)was then added followed by degassed ethylene glycol dimethacrylate (0.15mL). The mixture was then stirred at 800 rpm for 15 minutes to ensurecomplete dispersion of the quantum dots within the monomer mixture. Thesolution of 1% PVA (10 mL) was then added and the reaction stirred for10 minutes to ensure the formation of the suspension. The temperaturewas then raised to 72° C. and the reaction allowed to proceed for 12hours.

The reaction mixture was then cooled to room temperature and the beadedproduct washed with water until the washings ran clear followed bymethanol (100 mL), methanol/tetrahydrofuran (1:1, 100 mL),tetrahydrofuran (100 mL), tetrahydrofuran/dichloromethane (1:1, 100 mL),dichloromethane (100 mL), dichloromethane/tetrahydrofuran (1:1, 100 mL),tetrahydrofuran (100 mL), tetrahydrofuran/methanol (1:1, 100 mL),methanol (100 mL). The quantum dot-containing beads (QD-beads) were thendried under vacuum and stored under nitrogen.

2.2 Adsorbing Quantum Dots into Prefabricated Beads

Polystyrene microspheres with 1% divinyl benzene (DVB) and 1% thiolco-monomer were resuspended in toluene (1 mL) by shaking and sonication.The microspheres were centrifuged (6000 rpm, approximately 1 min) andthe supernatant decanted. This was repeated for a second wash withtoluene and the pellets then resuspended in toluene (1 mL).

InP/ZnS quantum dots as prepared above in Method 2 were dissolved (anexcess, usually 5 mg for 50 mg of microspheres) in chloroform (0.5 mL)and filtered to remove any insoluble material. The quantumdot-chloroform solution was added to the microspheres in toluene andshaken on a shaker plate at room temperature for 16 hours to mixthoroughly.

The quantum dot-microspheres were centrifuged to pellet and thesupernatant decanted off, which contained any excess quantum dotspresent. The pellet was washed (as above) twice with toluene (2 mL),resuspended in toluene (2 mL), and then transferred directly to glasssample vials used in an integrating sphere. The glass vials werepelleted down by placing the vials inside a centrifuge tube,centrifuging and decanting off excess toluene. This was repeated untilall of the material had been transferred into the sample vial. A quantumyield analysis was then run directly on the pellet, wet with toluene.

2.3 Incorporation of Quantum Dots into Silica Beads

0.4 mL of InP/ZnS core shell quantum dots capped with myristic acid(around 70 mg of inorganic) was dried under vacuum. 0.1 mL of(3-(trimethoxysilyl)propyl methacrylate (TMOPMA), followed by 0.5 mL oftriethylorthosilicate (TEOS) was injected to dissolve the dried quantumdots and form a clear solution, which was kept for incubation under N₂overnight. The mixture was then injected into 10 mL of a reversemicroemulsion (cyclohexane/CO-520, 18 ml/1.35 g) in 50 mL flask, understirring @ 600 rpm. The mixture was stirred for 15 mins and then 0.1 mLof 4% NH₄OH was injected to start the bead forming reaction. Thereaction was stopped the next day by centrifugation to collect the solidphase. The obtained particles were washed twice with 20 mL cyclohexaneand then dried under vacuum.

Example 1 Addition of Additive(s) to QD-Containing Beads

Any of the stability-enhancing additives set out hereinbefore can beadded to a quantum dot solution before processing the solution intobeads (e.g. mixed with a suitable monomer, crosslinker, and optionallyother ingredients), or added later to the pre-formed beads by incubationinto a solution of the additive, i.e. soaking, for a suitable period oftime. Soaking procedures for the addition of additives to pre-formedbeads involved adding 30 mg of dried quantum dot-containing beads to oneor more additive solutions in ethanol (additive concentration=1 mM). Thebeads were then incubated in this mixture for 30 mins and then dried byvacuum.

Example 2 Addition of Additive(s) to QD-Containing Beads Contained inLarger Beads

Inner beads containing quantum dots were mixed with one or moreadditives and then embedded within a larger bead. The final“bead-in-bead” material was then treated by soaking as described inExample 1.

Example 3

Coating Quantum Dot-Containing Silica Beads with Polymethylmethacrylate

25 mg powdered quantum dots-containing silica beads was dispersed aswell as possible in degassed methylmethacrylate (MMA). A photoinitiator,phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, was added to acrosslinker, trimethylolpropanetrimethacrylate (TMPTM), and dissolvedwhile the solution was degassed. The TMPTM crosslinker was then added tothe MMA and silica and the mixture agitated on a whirlmixer to ensurehomogeneous mixing of the monomer and crosslinker. The resulting slurrywas transferred to a syringe with a wide bore needle and thencontinuously agitated while being injected into 5 mL of degassed 2% PVAstirring at 1200 rpm. The suspension was then exposed to 365 nm UV lightfor 30 minutes. The mixture was stirred overnight and worked-up thefollowing morning by washing and centrifugation. Washes of 2×20 mL ofH₂O and 2×20 mL EtOH and centrifugation of 2000 rpm for 2 mins betweenwashes. The sample was finally dried under vacuum and purged with N₂.

What is claimed is:
 1. A method of making a quantum dot (QD)-containingbead-in-bead composition, the method comprising: encapsulating aplurality of QDs within a first bead material to provide a plurality ofprimary beads, each primary bead containing a plurality of QDsencapsulated therein; encapsulating a plurality of the primary beadswithin a second bead material to provide a plurality of secondary beads,each secondary bead containing a plurality of primary beads encapsulatedtherein; incorporating a stability-enhancing additive into either theprimary bead or into the secondary bead; and, coating the secondarybeads with an inorganic protective surface.
 2. The method of claim 1,wherein the QDs comprise ions selected from group 13 and group 15 of theperiodic table.
 3. The method of claim 1, wherein the first beadmaterial comprises resin, polymer, glass, sol gel, epoxy, silicone,acrylate, or silica.
 4. The method of claim 1, wherein the first beadmaterial is a (meth)acrylate polymer.
 5. The method of claim 1, whereinthe first bead material is silica.
 6. The method of claim 1, whereinencapsulating a plurality of QDs within a first bead material comprisessuspending the plurality of QDs in a solution of monomer andpolymerizing the monomer to provide the first bead material.
 7. Themethod of claim 1, wherein encapsulating a plurality of QDs within afirst bead material comprises suspending the plurality of QDs and aplurality of primary beads in a liquid and allowing the QDs to absorbinto the primary beads.
 8. The method of claim 1, wherein the primarybeads are about 20 nm to about 0.5 mm in diameter.
 9. The method ofclaim 1, wherein the inorganic protective surface comprises Al₂O₃. 10.The method of claim 1, wherein the second bead material comprisespolymer, resin, glass, sol gel, epoxy, silicone, or (meth)acrylate. 11.The method of claim 1, wherein the second bead material is a siliconepolymer.
 12. The method of claim 1, wherein the stability-enhancingadditive is a free-radical scavenger or a reducing agent.
 13. Abead-in-bead composition comprising: a plurality of primary beadsencapsulated within a secondary bead, wherein each primary beadcomprises a first bead material and a plurality of quantum dots (QDs)encapsulated within the primary bead, the secondary bead comprises asecondary bead material, the primary beads or the secondary beadcomprise a stability-enhancing additive, and the secondary beadcomprises an inorganic protective surface.
 14. The bead-in-beadcomposition of claim 13, wherein the QDs comprise ions selected fromgroup 13 and group 15 of the periodic table.
 15. The bead-in-beadcomposition of claim 13, wherein the first bead material comprisesresin, polymer, glass, sol gel, epoxy, silicone, acrylate, or silica.16. The bead-in-bead composition of claim 13, wherein the primary beadsare about 20 nm to about 0.5 mm in diameter.
 17. The bead-in-beadcomposition of claim 13, wherein the inorganic protective surfacecomprises Al₂O₃.
 18. The bead-in-bead composition of claim 13, whereinthe second bead material comprises polymer, resin, glass, sol gel,epoxy, silicone, or (meth)acrylate.
 19. The bead-in-bead composition ofclaim 13, wherein the second bead material is a silicone polymer. 20.The bead-in-bead composition of claim 13, wherein thestability-enhancing additive is a free-radical scavenger or a reducingagent.